Aluminum–Amido-Mediated Heterolytic Addition of Water Affords an Alumoxane

Abstract

Addition of the O−H bonds in water across the aluminum− nitrogen bond of a molecular aluminum−amido complex affords an alumoxane. The reaction of ( Ph I 2 P 2− )AlH (1) with water forms dimeric [( Ph HI 2 P − )AlH](μ-O) (2) under mild conditions. Upon reaction of 2 with excess water [( Ph HI 2 P − )Al(OH)](μ-O) (3) is formed with liberation of H 2 . T he addition of O−H bonds across the metal−ligand bonds of a metal complex, essentially an acid−base process, represents an alternative to oxidative addition of OH bonds at a single metal center. This elementary step has been shown to initiate the activation of alcohols toward reactions such as dehydrogenative coupling and amide formation. 1 The activation of water, via addition of the O−H bond across a metal− ligand bond, has also been reported by Milstein and co-workers, who showed formation of a ruthenium hydroxide complex. 2 Examples of heterolytic bond activation by group 13 metal complexes include activation of weak C−H bonds and of the N−H bonds in amines, 3,4 but we are not aware of examples of O−H cleavage.The bis(imino)pyridine (I 2 P) ligand system has been employed for its redox noninnocence in a variety of transition-metal complexes. 5 Reports of the chemical noninnocence of this ligand are rarer and include radical coupling to form new C−C bonds between ligands and the transfer of alkyl or hydride groups to various positions on the ligand backbone. 6 Budzelaar and co-workers showed that alkylation of a variety of I 2 P ligands by main-group-metal alkyls can occur at any site on the pyridine ring in addition to the imine carbons and that the selectivity is influenced by both the identity of the metal and the alkyl used. 7 In order to access the potential of metal−ligand cooperative pathways observed in other pincer ligand systems, it is necessary to design (I 2 P)M complexes that preferentially access two-electron acid−base reactions over radical or redox processes. 8 Due to their tendency of discrete complexes containing the Al−O−Al moiety (alumoxanes) to form oligomers, their formation is a synthetic challenge. 9 Most commonly, alumoxanes are formed through the interaction of water with Al−H, Al−R, or Al−X bonds (R = alkyl, X = halide), 10

Synthesis and characterization of bis(imino)pyridine complexes of divalent Mg and Zn

“…Complex 1 displays two reduction events at −1.09 V and −1.29 (4) 16.3329 (7) 9.5271 (3) 23.5150 (10) 32.0684 (10) b, Å 9.5686 (5) 17.6160 (3) 23.2052 (10) 11.8468 (4) 15.5547 (7) 34.4149 (11) c, Å 13.6954 (7) 22.1260 (4) 26.7476 (12) 21.7771 (8) 43.771 (2) 17.4423 (6) α,°108.253(2) …”

Catalysis by Aluminum(III) Complexes of Non‐Innocent Ligands

“…Modestly acidic substrates, such as anilines and water (henceforth denoted generally as RXH), protonate the I 2 P 2− ligand of ( Ph I 2 P 2− )Al(THF)H at the amido donor of the ligand and result in heterolytic cleavage of the XH bonds and formation of ( Ph HI 2 P 1− )Al(XR)H complexes (Scheme ) 23. 27…”

Reversible OH-bond activation and amphoterism by metal–ligand cooperativity of calix[4]pyrrolato aluminate

“…[9][10][11][12] Irreversible ELC reactivity of main-group elementbased systems with OH-substrates was reported for a few examples: Milstein and coworkers found a 1,2-dihydrodiazaborolopyridine capable of adding benzoic acid through boron-ligand cooperation. 10 The Berben group observed aluminum-ligand cooperative addition of phenol, benzyl alcohols, 13 water, 14 and formic acid 15 to their bis(iminopyridine)-ligated aluminum hydride. 16 Other examples of aluminum-based cooperative OHaddition reactions were provided by Fedushkin 17 and recently by Zhu.…”